1. Field of the Invention
The present invention relates to a noise filter and a method of making the same, and particularly to a four-terminal type noise filter and a method of making the same.
2. Description of the Prior Art
With the development of electronic technology, electronic circuits have been used in a great variety of fields. It is desirable that such electronic circuits function in a stable and reliable state without that state being affected by an external occurrence such as noise on the power line.
However, direct or indirect effects on the electronic circuits cannot be avoided. This leads to a problem in that various electronic devices utilizing electronic circuits frequently malfunction.
Many electronic circuits use a switching regulator type power supply as a source of direct current. Due to transient currents on switching or from load variations resulting from the internal switching operations of the digital ICs, noises having various frequency components are frequently produced on the power line of the switching regulator. These noises are transmitted to the other circuit components in the same electronic device on the power line or by radiation to create a malfunction or a reduction of the signal to noise ratio. Furthermore, they may create a malfunction in any one of the other electronic devices used in the vicinity of the electronic device in which the noises are produced.
In order to minimize the noise, various types of noise filters have previously been used in the electronic circuits. Since the use of high-performance electronic instruments is increasing, noise regulations are increasingly being tightened. It is therefore desirable to have a small-sized and high-performance noise filter which can also minimize noise.
For such a purpose, a noise filter having an equivalent circuit as FIG. 14B of the accompanying drawings is currently used. In FIG. 14A a conventional four-terminal noise filter is shown. This filter comprises a magnetic core 10, a pair of windings 12 and 14 wound about the core 10, and capacitors 16 and 18 connected in parallel to each other between the corresponding ends of the windings 12 and 14. Since such an arrangement requires the core 10, the noise filter itself is of increased size and manufacturing cost in comparison to the other electronic components of the device have been reduced in size and weight.
From an inspection of FIG. 14B, it can further be seen that this prior art four-terminal type noise filter is a centralized constant type. Such an arrangement cannot eliminate various noise, and, more particularly, common mode noise as in switching surges and normal noise such as ripple noise. Additionally, if the values of the capacitors or coils are increased an undesirable ringing may occur.